1. Technical Field
This invention relates to speech communication systems and, more particularly, to systems and methods for digital speech coding.
2. Related Art
One prevalent mode of human communication involves the use of communication systems. Communication systems include both wireline and wireless radio systems. Wireless communication systems electrically connect with the landline systems and communicate using radio frequency (RF) with mobile communication devices. Currently, the radio frequencies available for communication in cellular systems, for example, are in the frequency range centered around 900 MHz and in the personal communication services (PCS) frequency range centered around 1900 MHz. Due to increased traffic caused by the expanding popularity of wireless communication devices, such as cellular telephones, it is desirable to reduced bandwidth of transmissions within the wireless systems.
Digital transmission in wireless radio communications is increasingly being applied to both voice and data due to noise immunity, reliability, compactness of equipment and the ability to implement sophisticated signal processing functions using digital techniques. Digital transmission of speech signals involves the steps of: sampling an analog speech waveform with an analog-to-digital converter, speech compression (encoding), transmission, speech decompression (decoding), digital-to-analog conversion, and playback into an earpiece or a loudspeaker. The sampling of the analog speech waveform with the analog-to-digital converter creates a digital signal. However, the number of bits used in the digital signal to represent the analog speech waveform creates a relatively large bandwidth. For example, a speech signal that is sampled at a rate of 8000 Hz (once every 0.125 ms), where each sample is represented by 16 bits, will result in a bit rate of 128,000 (16×8000) bits per second, or 128 Kbps (Kilo bits per second).
Speech compression reduces the number of bits that represent the speech signal, thus reducing the bandwidth needed for transmission. However, speech compression may result in degradation of the quality of decompressed speech. In general, a higher bit rate will result in higher quality, while a lower bit rate will result in lower quality. However, speech compression techniques, such as coding techniques, can produce decompressed speech of relatively high quality at relatively low bit rates. In general, coding techniques attempt to represent the perceptually important features of the speech signal, with or without preserving the actual speech waveform.
One coding technique used to lower the bit rate involves varying the degree of speech compression (i.e., varying the bit rate) depending on the part of the speech signal being compressed. Typically, parts of the speech signal for which adequate perceptual representation is more difficult or more important (such as voiced speech, plosives, or voiced onsets) are coded and transmitted using a higher number of bits, while parts of the speech signal for which adequate perceptual representation is less difficult or less important (such as unvoiced, or the silence between words) are coded with a lower number of bits. The resulting average bit rate for the speech signal may be relatively lower than would be the case for a fixed bit rate that provides decompressed speech of similar quality.
These speech compression techniques have resulted in lowering the amount of bandwidth used to transmit a speech signal. However, further reduction in bandwidth is important in a communication system for a large number of users. Accordingly, there is a need for systems and methods of speech coding that are capable of minimizing the average bit rate needed for speech representation, while providing high quality decompressed speech.